Differing methods are available in prior art to protect the surfaces of metal materials against corrosive environmental factors. Coating of the metal workpiece to be protected using a finish made of a different metal is a widespread and established method in technology. The coating metal can in the process behave either more nobly or less nobly electrochemically in the corrosive medium than the basic metal of the work piece. If the coating metal behaves less nobly, then it operates in the corrosive medium as a galvanic anode towards the base metal (cathodic corrosion protection). Thus, although this protective function linked to the creation of the coating metal's corrosion products is desirable, the coating's corrosion products however often lead to undesirable decorative and often also functional impairment of the work piece. In order to reduce the corrosion of the coating metal or to prevent it for as long as possible, so-called conversion layers are used, especially on cathodic protecting base, coating metals, such as zinc or aluminium, for instance and their alloys. Here one is dealing with reaction products of the base coating metal largely insoluble in aqueous media across a broad pH range with the treatment solution. Phosphate and chromate coatings are examples of so-called conversion coatings.
The surface to be treated is plunged into an acid solution containing chromium(VI) ions (cf. EP 0 553 164 A1) in the case of chromate coatings. If, for example, the surface is zinc, then part of the zinc dissolves. Chromium(VI) is reduced to chromium(III) under the reducing conditions prevailing which is eliminated due to the development of hydrogen as chromium(III) hydroxide or as poorly soluble p-oxo bridged or p-hydroxide bridged chromium(III) complex in the alkaline surface film. Poorly soluble zinc chromate(VI) is formed in parallel. A densely continuous conversion coating is formed on the zinc surface which protects very well against a corrosive attack by electrolytes.
However, chromium(VI) compounds are acutely toxic and highly carcinogenic, so that a replacement for the process which accompanies these compounds is needed.
In the meantime, a multitude of processes have established themselves as a replacement for chromatising processes with hexavalent chromium compounds using different complexes of trivalent chromium compounds (cf. DE 196 38 176 A1). As the corrosion protection obtained this way is inferior as a rule to the process working with hexavalent chromium, a sealing is often applied in addition to the surface of the work piece. Sealing such as this can be carried out based, for example, on inorganic silicates, organofunctional silanes, organic polymers and hybrid systems exhibiting both organic and inorganic constituents as film formers. The disadvantage of this additional step in the process is the occurrence of run-off drops when coating work pieces manufactured on a frame and/or the bonding of coated bulk products. Problems such as the dimensional stability of threads and the like arise in addition, which are accompanied by the layer thickness of these sealings.
Attempts which combine the corrosive protection properties of coatings made from chromiferous passivations and subsequent sealings in a single layer are described in prior art:
The document EP 0 479 289 A1 describes a chromatising process in which the substrate is plunged into a treatment solution containing a silane coupling agent in addition to chromium(VI) and chromium(III) ions, hydrofluoric acid and phosphoric acid.
The patent EP 0 922 785 B1 describes a treatment solution and a process for producing protective layers on metals where the surface to be protected is coated with a treatment solution containing chromium(III) ions, an oxidant, an oxyacid or an oxyacid salt of phosphorous or a corresponding anhydride. Further, this treatment solution can contain a monomeric silane coupling agent.
A treatment solution for increasing the corrosion protection of substrates is described in EP 1 051 539 B1 containing phosphoric acid, hydrofluoric acid, colloid silicon dioxide and a monomer epoxy functionalised silane.
WO 2008/14166 A1 describes a treatment solution for the production of corrosion protection layers. In addition to zinc ions, this treatment solution contains phosphoric acid or acid phosphates, organic or inorganic acid ions, which contain one of the elements boron, silicon, titanium or zirconium, trivalent chromium ions and an inorganic or organic peroxide as an oxidant.
WO 97/15700 A1 describes a treatment solution for the production of corrosion protection layers. The treatment solution contains hydrolysed silanes and phosphoric acids and is free of chromium ions and chromium containing compounds.
The treatment solutions described in prior art exhibit the following disadvantages: Either they contain toxic substances, such as chromium(VI) ions and hydrofluoric acid or monomeric silanes. Well-controlled hydrolysis and condensation of monomeric silanes cannot be carried out in matrixes such as these and therefore lead to varying properties in the resulting coatings.